File I/O in Visual Basic

2 types of files exist:

• sequential plain text; read and write 1 line at a time
• random access read/write in any order; lines(records) must all be equal length

File Operations

Normally, use the common dialog control to present to present the file open or file save as dialog box.

You can customize using

• file list box
• directory list box
• drive list box

Sequential Files

Bytes stored in file on disk

Displayed on screen:

@echo off

set ...

Carriage Return = ASCII 13

Line Feed = ASCII 10

Opening & Closing Sequential Files

OPENING A FILE ...

open "c:\config.sys" for input as #1

or

doc$ = "c:\config.sys"

open doc$ for input as #1

Because OPEN for OUTPUT destroys existing files ...

if dir$("c:\config.sys") <> "" then

if inputbox("c:\config.sys already exists. Delete?")<> "Yes" then

exit sub

end if

end if

open "c:\config.sys" for output as #2

CLOSING A FILE

close #1

Reading from a Sequential File

LINE INPUT #fileNumber,stringvariable

open filevar for input as #1

do while not EOF(1)

line input #1, linevar

/* do stuff */

loop

close #1

Writing to a Sequential File

PRINT #fileNumber, expression | expression | ...

If semicolons are used to separate expressions THEN no space is between record fields

If commas are used to separate expressions THEN skip to next print field (up to 14 chars)

Example using Sequential Files: Program to edit config.sys

3 command buttons:

• open btnOpen
• save btnSave
• quit btnQuit

Multiline text box with both scrollbars; txtContent

General declarations:

• dim textchanged as integer
• dim configfound as integer
• dim crlf as string

The procedures:

sub form_load()

textchanged = false

configfound = false

crlf = chr$(13) + chr$(10)

end sub

 

sub btnOpen_click()

dim cline as string

if configfound then

beep

exit sub

end if

open "c:\config.sys" for input as #1

do while not EOF(1)

line input #1, cline

txtContent.text = txtContent.text + cline + crlf

loop

close #1

configfound = TRUE

end sub

 

sub btnSave_click()

If not (configfound AND textchanged) then

beep

exit sub

end if

open "c:\config.sys" for output as #1

print #1, txtContent.text

close #1

end sub

 

sub btnQuit_click()

end

end sub

 

sub txtContent_change()

textchanged = TRUE

end sub

Random Access Files

open filename for random as filenumber len = recordlength

• The Data and Records

type Weasel

name as string * 32

color as string * 12

weight as integer

length as integer

birthdate as double

end type

Fix size of string var’s

Sum up record length: 32 + 12 + 2 + 2 + 8 = 56

Reading & Writing to Random Access Files

• get #fileNumber, [recordnumber], variable
• put #fileNumber, [recordnumber], variable